When scheduling a unit it is important to know if the unit is available and online at the time.
The easiest way to check this it to review the units from within the scheduling application. A unit that is offline will have a yellow triangle beside it:
The alert triangles should be visible in a number of locations when you are creating a new schedule:
List view of the schedule function:
General alerts on the page.
If you select your bays/paddocks from the map you should see these white triangles.
If you see these triangles on any of the units intended for scheduling, it is important to ensure the unit is working correctly, prior to scheduling the unit. A unit that is not online or has an issue can cause problems with a schedule.